Among the conventional systems for detecting unauthorized intrusion into a given area are systems in which acoustoelectric transducers monitor several remote areas and convert sounds occurring in these areas into electrical signals which are transmitted to a centrally-located monitoring unit for review by security personnel. Such areas include, for example, entrances to large department stores, office buildings, high-rise apartment buildings, or factories. When a sound at a given remote area exceeds a predetermined threshold value, a transducer means transmits the sound to the monitoring unit having an alarm indicating means such as an audio alarm and/or visual light which is actuated to alert the security personnel.
While it is possible to provide very elaborate and sophisticated electronic intruder alarm systems for the security personnel, such systems are generally too expensive to construct for practical use in many applications. Moreover, such complicated systems are generally not operated proficiently by most security personnel because of a lack of understanding, confidence, or even ability required to operate complicated electronic apparatus. Centralized systems of this kind should therefore be relatively simple and foolproof in their operation as well as highly immune to sabotage by sophisticated criminals.
Another frequent shortcoming of many present systems is their lack of flexibility to accommodate unusual situations without triggering the alarms. For example, the presence of cleaning personnel, traffic noise, or nearby construction may result in one area having significantly higher background noise than any other area and which may trigger an alarm when there has been no unauthorized intrusion into that area. Preferably, a centralized system should permit simultaneous monitoring of all of the remote areas for individual noises and sound levels. This facilitates distinguishing rather quickly between an unauthorized intrusion into an area, which requires immediate and cautious attention, and a noise which upon closer listening can be identified as not resulting from an intrusion into the area by a burglar or other unauthorized persons.
As space is costly in many of these locations, the systems should be small and compact and readily installed with simple cables. Also, detection of sabotage should be provided as should tests for checking the operation of the system.
Accordingly, an object of the present invention is to provide an improved centralized system of the foregoing kind for monitoring a plurality of remote areas.